DNA topoisomerase II (Topo II) is crucial for resolving topological problems of DNA and plays important
roles in various cellular processes, such as replication, transcription, and chromosome segregation.
Although DNA topology problems may also occur during DNA repair, the possible involvement of
Topo II in this process remains to be fully investigated. Here, we show the dynamic behavior of
human Topo IIβ in response to DNA double-strand breaks (DSBs), which is the most harmful form of
DNA damage. Live cell imaging coupled with site-directed DSB induction by laser microirradiation
demonstrated rapid recruitment of EGFP-tagged Topo IIβ to the DSB site. Detergent extraction
followed by immunofluorescence showed the tight association of endogenous Topo IIβ with DSB sites.
Photobleaching analysis revealed that Topo IIβ is highly mobile in the nucleus. The Topo II catalytic
inhibitors ICRF-187 and ICRF-193 reduced the Topo IIβ mobility and thereby prevented Topo IIβ
recruitment to DSBs. Furthermore, Topo IIβ knockout cells exhibited increased sensitivity to bleomycin
and decreased DSB repair mediated by homologous recombination (HR), implicating the role of Topo IIβ
in HR-mediated DSB repair. Taken together, these results highlight a novel aspect of Topo IIβ functions
in the cellular response to DSBs.